This invention relates in general to half duplex transceiver systems, and more particularly, half duplex transceiver systems making use of complimentary switched amplifiers.
High frequency transceivers, used for half duplex communications applications, commonly make use of a single antenna for both a transmit mode and a receive mode of transceiver operation. A single antenna is appropriate for use in both transmit and receive modes when the frequencies associated with transmit and receive modes are the same or close enough to the same so as to fall within the useful bandwidth of the antenna. Using a single antenna for both transmit and receive functions minimizes the number of antennas required for transceiver operation, and generally results in a physically smaller system than a transceiver using separate antennas for transmit and receive functions.
Use of a single antenna for both transmit and receive modes requires some method of directing RF to the antenna from a transmit path in the transmit mode. Use of a single antenna also requires that the method direct RF to a receive path from the antenna in the receive mode.
The method should also isolate the receive path from the transmit path and the antenna in the transmit mode. Isolating the receive path from the transmit path and the antenna is important during transmit mode when transmit power is directed to the antenna from the transmit path. Transmit power levels can often be high enough to damage sensitive receiver elements, if not sufficiently isolated from the receive path. The method should also isolate the transmit path from the receive path when receive power is directed from the antenna to the receive path. Isolating the transmit path from the receive path prevents the transmit path from loading the receive path, and thus pulling the input impedance seen by the receive path away from an optimal noise impedance.
One common method used to properly direct RF to and from the antenna utilizes a single pole double throw electronic switch which selects either the receive path or the transmit path for coupling with the antenna. An electronic switch can be designed to provide good isolation characteristics to protect the receiver from RF transmit power directed to the antenna. An electronic switch provides isolation by presenting a high impedance in the path from the antenna to the transmitter in receive mode and a high impedance in the path between the antenna and the receiver in transmit mode.
Additional protection of the receiver can be obtained by electronically disabling the receiver through removal of DC bias. This not only protects the sensitive receiver elements, but conserves DC power.
One detrimental effect of inserting a switch between the antenna and both the transmit path and the receive path is that the insertion loss of the switch degrades system performance for both the transmit mode and the receive mode. In transmit mode, the switch loss reduces the actual power available for transmission. The switch loss also limits the amount of power which can be transmitted. Most of the power lost through a switch is absorptive loss, which when transmit power is high, results in excessive heat. In receive mode, the switch loss raises the "noise floor" of the entire receiver and reduces the overall receiver sensitivity.
Along with the performance degradation resulting from placing a switch between the antenna and both the transmit path and the receive path, in integrated circuit form, a switch often utilizes long transmission lines with electrical lengths of significant percentages of a wavelength. The long transmission lines occupy considerable area on an integrated circuit and are thus expensive.
A method of providing half duplex transceiver functions using a single antenna, without the performance degradation effects of switch insertion loss between the antenna and the transmit or receive paths would enhance overall system performance.